In a W-CDMA network that is a conventional typical radio access system, two-stage access barring is performed as illustrated in FIG. 12. A protocol architecture of a radio interface in the W-CDMA includes a physical layer (L1), a data link layer (L2), and a network layer (L3). When a mobile device tries to access the network, first, whether an access to a cell is allowed is determined in the L3 layer of the mobile device. If allowed, next, whether use of a random access channel (RACH) is allowed is determined in the L2 layer of the mobile device.
The allowance of an access to a cell specifically means the allowance of start of a procedure of establishing a radio resource control (RRC) connection, and the allowance of use of a RACH specifically means the allowance of start of a physical RACH (PRACH) transmission procedure in the L1 layer.
The access barring in the L3 layer in the W-CDMA (see Non-Patent Literature 1) is typically performed such that a radio network control device (RNC) writes barring information in a system information block (SIB) to be broadcasted to mobile devices in accordance with the degree of congestion of the network controlled by the RNC itself. To be specific, the RNC writes, as an access class (AC) barring status, whether an access is allowed (0) or an access is denied (1) for each AC in a part called an SIB type 3 (SIB3).
Here, the access class includes normal type ACs (AC=0 to 9) and special type ACs (AC=11 to 15). As the special type ACs, for example, AC=11 is allocated to a mobile device for network operators, AC=12 is allocated to a mobile device for police services, AC=13 is allocated to a mobile device for governmental use, and AC=14 is allocated to a mobile device for emergency services. A normal type AC may be additionally allocated to the mobile device to which the special type AC is allocated.
Therefore, typically, an AC barring status in which accesses of all of the special type ACs are allowed (0), accesses of a part of the normal type ACs are allowed (0), and accesses of the rest are denied (1) is written in the SIB3. If there are 10 normal type ACs, a percentage of mobile devices barred from accessing can be changed by 10%.
For example, as illustrated in FIG. 13, nine ACs set with access-allowed (0) and one AC set with access-denied (1) are designated for 10% barring, while seven ACs set with access-allowed (0) and three ACs set with access-denied (1) are designated for 30% barring. At this time, keeping an access of the same AC denied for a long time is problematic from the standpoint of fairness of communication service. Therefore, the AC, an access of which is denied, is circulated among AC=0 to 9 with time.
The mobile device refers to the barring status corresponding to an AC to which the device itself belongs from the most recently broadcasted SIB3 before starting a establishment procedure of the RRC connection, and does not start the establishment procedure when an access being denied (1) is designated. When an access being allowed (0) is designated, the mobile device starts the establishment procedure, and proceeds in control in the L2 layer described below, accordingly.
The access barring in the L2 layer in the W-CDMA (see Non-Patent Literature 2) is such that, first, information indicating a mapping of the access class (AC) and an access service class (ASC) is described in an SIB type 5 (SIB5), as illustrated in FIG. 12, and is broadcasted to the mobile device.
For example, it is possible to correspond most preferential ACs (for example, AC=12 and 14) from among the special type ACs to ASC=0, the rest of the special type ACs (for example, AC=11 and 13) to ASC=1, and the normal type ACs (AC=0 to 9) to ASC=2. As a simple example, when the ASC has two types of 0 and 1, preferential ACs (for example, AC=12 and 14) from among the special type ACs are made corresponding to ASC=0, and the rest of the special type ACs and the normal type ACs (for example, AC=0 to 9, 11, 13, and 15) are made corresponding to ASC=1.
Then, a persistence value (Pi) that determines how much percentage of mobile device can use the RACH in each ASC(i) corresponding to the AC is determined based on a persistence level (N) described in an SIB type 7 (SIB7) and broadcasted. N is a natural number of 1 to 8, and P(N)=2−(N−1) is determined. P0 of ASC=0 is 1, P1 of ASC=1 is P(N), P2 of ASC=2 is s2P(N) (si is a number from 0 to 1, and is broadcasted with the SIB5).
FIG. 14 illustrates an operation of a mobile device in examples where the ASC has two types of 0 and 1. First, the mobile device checks which ASC the AC corresponds to from the mapping, where the own device belongs to the AC (S910), obtains the persistence value P(N) using N if ASC=1 (S920), and sets P=1 if ASC=0 (S930). For example, P (N)=½ if N=2, and P (N)=¼ if N=3.
Then, the mobile device generates a random number in a range of 0 to 1 (S940), and compares the random number with P(N) (S950). The mobile device determines an access is denied if the random number is larger than P(N) (S960), and determines an access is allowed if the random number is smaller than P(N) (S970). In a case where P=1 is set, when the random number is compared with P, it is always determined that an access is allowed (the dotted-line arrow in the drawing). The mobile device interprets the RACH can be used when having determined that an access is allowed, and starts a PRACH transmission procedure.
With the above-described control, 100% of the mobile devices having ASC=0, 25% of the mobile devices having ASC=1 in the example of N=3, and 12.5% of the mobile devices having ASC=2 where s2=0.5 can access the network using the RACH. A value of N that is the basis for determining the percentage is typically determined by a radio base station (NodeB) that performs measurement of a load in a cell of the own station, and is described in the SIB7.
As described above, in the network of W-CDMA, the congestion of the network has been overcome by a mechanism in which the L3 layer and the L2 layer perform two-stage access barring. However, in a next-generation Long Term Evolution (LTE), the mechanism is integrated into access barring in the L3 layer.
Note that the access barring in the L3 layer in the W-CDMA circulates the AC to be barred with time, as illustrated in FIG. 13. Therefore, it is necessary to periodically change the AC barring status broadcasted with the SIB3. In a case where the AC barring status is changed in every several tens of seconds in order to maintain the fairness of communication service, for example, when a paging message that indicates the change of the content of the SIB3 is repeatedly transmitted during a predetermined period so that the change is passed on to all of the mobile devices, it soon gets to a next change point, and there is a high possibility of almost steadily transmitting the paging message. If so, there is a problem that a mobile device that needs to almost steadily receive the paging message may burn battery power.
To solve this problem, in the access barring in the L3 layer in the LTE (see Non-Patent Literature 3), the appropriateness of an access is determined by comparison with a random number generated by the mobile device by following the access barring in the L2 layer in the W-CDMA.
This is because, when 30% barring is desired, for example, if a value indicating “70%” is broadcasted at the beginning of a barring period, the mobile device then compares the value with a random number, so that it is determined that an access is allowed by the probability of 70% (70% from among the mobile devices in the cell), and it is determined that an access is denied by the probability of 30% (30% from among the mobile device in the cell), whereby frequent change of the broadcast information becomes unnecessary. Note that, for this purpose, the LTE broadcasts information for designating values at 5% intervals from 0 to 95% instead of broadcasting the persistence level (N) like the W-CDMA.
In following the mechanism of the access barring in the L2 layer in the W-CDMA in this way, a mechanism in which an access is always allowed if ASC=0 and the appropriateness of an access is determined according to the comparison with a random number if ASC=1 is introduced as it is. Therefore, in the LTE system, the operation illustrated in FIG. 15 is performed in the mobile device.
In the access barring of the LTE, whether each AC corresponds to ASC=0, in which the AC is not subjected to barring, or to ASC=1, in which the AC is subjected to barring, is broadcasted as information of ac-BarringForSpecialAC. Note that the normal type ACs (AC=0 to 9) cannot correspond to ASC=0, and therefore, broadcasted information is information regarding special type ACs (AC=11 to 15).
That is, in the LTE system, information of designating no barring (0) for preferential ACs (for example AC=12 and 14) from among the special type ACs, and of designating barring (1) for the rest of the special type ACs and the normal type ACs (for example, AC=0 to 9, 11, 13, and 15) is broadcasted.
Further, in the access barring of LTE, the mobile device having an AC subjected to barring broadcasts information of ac-BarringFactor as information for designating values to be compared with a random number (values at 5% intervals from 0 to 95%).
The mobile device that has received the information then proceeds to Yes at S1010 and determines accesses of the ACs among the special type ACs, in which 0 is designated (for example, AC=12 and 14), are allowed (S1050), and proceeds to No at S1010 and generates random numbers of the ACs from among the special type ACs, in which 1 is designated, and the normal type ACs (for example, AC=0 to 9, 11, 13, and 15) (S1020) by the ac-BarringForSpecialAC, as illustrated in FIG. 15. If the generated random number is smaller than the value designated by the ac-BarringFactor (Yes at S1030), it is determined that an access is allowed (S1050), and if the generated random number is larger than the value designated by the ac-BarringFactor (No at S1030), it is determined that an access is denied (S1040).